1. Field of the Invention
The present invention relates to a method for laser welding multiple sheets of steel together. More particularly, the present invention relates to a method for performing a laser beam welding operation to molecularly bond two pieces of zinc coated steel together.
2. Description of Related Art
Conventionally, laser beam machines are known to be employed, not only for cutting flat or otherwise drawn sheet metal along given cutting lines, but also for spot or seam welding sheets together.
According to general practice, two steel sheets for laser welding are held together contacting each other as tightly as possible along the entire weld area by means of grips, so as to ensure, among other things, maximum thermal conduction between the sheets. The sheets are then subjected to a laser beam, which welds the sheets together by smelting the metal in the weld area swept by the beam.
While the aforementioned method has proved particularly effective for welding bare sheet steel, i.e. having no covering material protecting it against external agents, it proves inadequate when welding together metal sheets protected against external agents by using a layer of coating of low-vaporizing-temperature materials. The term xe2x80x9clow-vaporizing-temperature materialxe2x80x9d is intended to mean material, such as zinc, that has a melting and vaporizing temperature considerably below that of iron.
During laser welding, the energy from the laser beam penetrates through the first piece into the second piece of steel, heating portions of the two pieces to a sufficiently high temperature so that they melt and coalesce together to form a lap joint. However, the zinc material on the abutting surfaces of the first and second pieces violently vaporizes into a gaseous state and tends to separate the upper and lower portions of the weld zone and/or expand through the weld zone toward the laser beam to create porosity in the final weld.
If two thus protected metal sheets using a material such as zinc or a similar material are welded together using the same technique employed for welding bare sheet, the resulting welds invariably prove uneven and riddled with craters, faults which, involve high-cost follow-up machining for their removal.
Methods are known to eliminate these imperfections during the welding process when welding two sheets of metal, the metal being of the type having associated gases tending to be trapped and expand in the weld zone, e.g., vaporized zinc, during welding due to heat from the laser. One method adds to the standard laser beam a surrounding stream of pressurized shield gas effective to create a pressure at the surface of the weld zone sufficient to force the molten metal of the two sheets together and force the expanded associated gases out of the weld zone in a direction away from the laser beam, whereby a non-porous weld may be created.
Another method for welding galvanized material discloses a low vapor pressure mild steel core and a high vapor pressure rich zinc coating including the steps of arranging components of such galvanized material in juxtaposed relationship at a lapped joint and applying a high density laser energy beam along the lapped joint as a weave pattern. The weave pattern has a width great enough to bridge the lapped joint and a weave pattern frequency, which forms a predetermined weld pool between the components. Here, the lapped joint and weave pattern combine to define a vapor pressure relief path so that the weld pool will not be disrupted during the application of the high density laser energy beam to the galvanized components.
It is also known to laser weld steel sheets that have a thin corrosion protective coating of zinc with a method where the steel sheets are positioned vertically. A laser beam, which is positioned normal to the sheets, is then applied to the sheets to melt the material of the sheets and create a weld. During the welding, the sheets and laser beam are moved vertically relative to each other such that laser heating of the material creates a cavity. Thus, liquid or molten material flows vertically downwardly by gravity to elongate the cavity and thereby facilitate the escape of zinc vapors from the cavity.
Similarly, it is known to use a pulsed laser beam when laser welding steel sheets that have a thin corrosion protective coating of zinc to melt the material of the sheets and create a weld. During welding, the laser beam is pulsed ON and OFF and the sheets and pulsed laser beam are moved vertically relative to each other such that laser heating of the material creates a cavity. Here again, liquid or molten material flows vertically downwardly by gravity to elongate the cavity and thereby facilitate the escape of zinc vapors from the cavity.
It is also known to alter the shape of the sheets, the location of the clamp, and the placement of the weld in order to allow external communication between a protective layer and the sheets in the vicinity of the weld area.
Thus, the prior art fails to provide adequate disclosure of the relationship of the sheet shape and the weld location relative to the physical characteristics of the sheets.
In view of the above-mentioned drawbacks, there is a need for a specific geometrical relationship between two zinc-coated sheets of materials, the laser weld location and the geometric shape of the sheets.
The present invention is directed to a method to perform laser welding for two pieces of metal (preferably, zinc coated steel) together. The method includes providing a first and a second piece of metal in an overlapping relationship. The first piece is curved or otherwise diverges from the second piece at or proximate the location where the laser weld is to be performed. A zone for the laser welding operation is defined between first and second edges and has a predetermined width. The laser weld is then preformed in that zone.